The present invention relates generally to wellhead equipment for use in completion of oil or gas wells, and more particularly to an expandable load ring carried by an annular weight support member for expansion into a recessed load support groove in a surrounding structural support member. The weight support member may be, for example, a casing hanger, and the surrounding structural support member may be, for example, a casing head.
In the modern-day completion of conventional oil or gas wells, it is not uncommon for the casing program to include several strings of intermediate casing between the oil string casing and the surface or conductor casing, as well as one or more tubing strings. Typically, the surface or conductor casing is first run and cemented in the well bore, and the lowermost or surface casing head mounted on its upper end. Drilling then continues and an additional casing head is installed. The casing is set in a hole drilled at the rig, and a blowout preventer, or BOP, is located over the casing and hole being drilled. Installation of each additional casing head requires the removal of the blowout preventer. Then, after all casings are in place, a similar procedure is followed for the tubing head and tubing strings. Usually, the casing strings are each suspended from hanger equipment supported in a separate casing head. Similarly, the tubing strings are usually supported in a separate tubing head.
Installing separate heads for each casing string after the surface casing is installed is time-consuming, expensive, and potentially very dangerous. Each time a new casing head is installed on top of the wellhead equipment already present, the blowout preventer equipment must be removed. Typically, the blowout preventer stack is lifted and suspended while rig operators, working under it, install the new head. Since the BOP stack can weigh fifty tons or more, an accident or equipment malfunction or failure while the stack is suspended, causing partial or complete collapse of the stack, can cause serious injury or death to operators, as well as costly damage to equipment and lost rig time. When the new head has been installed and the BOP stack lowered for reinstallation, or another size or type of BOP equipment is installed to accommodate larger casing hangers, that too can be a dangerous and costly operation, especially if an equipment malfunction or failure were to occur. Operators exhibiting less than complete care and attention have lost fingers or hands during the BOP stack reinstallation operation, even when all equipment operates normally and as expected. The hazards inherent in working with equipment of this size and weight are amplified by the non-ideal working conditions on the rig; equipment, rig structure, and personnel are frequently covered by oil, grease, mud, or the like.
In addition to exposing operators to the dangers of mishandled equipment and accidents, removal of the BOP stack to install a new head is inherently dangerous, since during that time the well, and consequently the rig and operators, are unprotected against potentially catastrophic blowouts.
At best, when all equipment operates normally and without any accidents or the like, installation of a new head with BOP stack removal and reinstallation can take anywhere from about four to about twelve hours. For highly expensive rig crews and equipment, this operation can thus amount to a prohibitive expense.
In order to avoid some of the dangers and expense of installing a separate casing head for each casing, sometimes casing heads are used which can accommodate more than one casing string. This is sometimes done by providing the head with two annular support shoulders, with the larger ID shoulder simply being stepped back from the smaller one. The first casing is hung from the lower end of the casing head, and the second casing slides through the first casing, but has a larger flange which rests in the casing head above the first casing hanger. A separate casing hanger rests on each shoulder, suspending casing therefrom. With conventional heads of this type having a plurality of stepped-back annular shoulders, however, it is very difficult, if not entirely impossible, to provide for more than two such annular support shoulders while remaining within API requirements for the minimum shoulder bearing areas for the flange sizes of the hangers to be supported thereon. For any casing head, the minimum ID permissible in the casing head bore will be determined by the OD of the largest casing suspended from the bottom of the head. Thus, the total bearing area available for stepped-back annular support shoulders in conventional heads will be limited by this minimum head ID. The minimum bearing area for each annular support shoulder is determined such that it will be capable of supporting not only the weight of the hanger and its suspended casing, but also any load applied to the hanger's cross-sectional area from internal wellhead pressures. As hanger flange sizes increase for smaller diameter casings suspended above the lowermost casing, the potential pressure load increases, without regard to whether the suspended casing weight for the smaller casings might be less than the larger. Therefore, the minimum bearing area required for support shoulders for smaller casings will still be substantial, in order to support expected string weights and pressure loads.
There simply is not enough available bearing area in conventional heads for three or more stepped-back annular shoulders while remaining within API requirements; if three or more such shoulders are provided, one or more of them will not have the minimum specified bearing area for hangers of that flange size. In addition, for conventional heads as just described, as larger flange sizes are provided for the hangers to suspend smaller casing strings, it becomes more difficult, if not impossible, to run such hangers through the often restricted bores of the blowout preventer equipment mounted above the head. Accordingly, even if three or more annular stepped-back support shoulders were to be provided in the head, the conventional hangers for the smaller casing strings probably could not be run through the BOP stack, thereby necessitating BOP stack removal and eliminating the advantages of suspending the third and smaller strings from the same head in the first place.
Another approach to providing a multi-string head which has met with some success in the oil and gas industry is exemplified by the so-called "speed head" and hanger apparatus described, for example, in U.S. Pat. No. 3,438,654, issued Apr. 15, 1969, to J. G. Jackson, Jr., et al. While head and hanger equipment of that type is capable of suspending a plurality of casing and tubing strings from a single head, such equipment is inordinately complex, difficult to manufacture and relatively difficult to install, and expensive. Yet another approach to a multi-string head which has met with some success is the split load shoulder, such as one offered by Gray Tool Company, for example. The split load shoulder is typically a hinged circular ring which is installed in the head in expanded condition, and is actuated by a crank handle mounted on the outside of the head. Rotation of the crank handle reduces the ID of the split load shoulder, providing the required bearing area. Although probably less complicated than the speed head and hanger, the split load shoulder and accompanying head is still a more complex, expensive, and difficult system to manufacture and install than that of the present invention. Yet another multi-string system which has been proposed is the new "Cam Forge" system offered by Cameron Iron Works, which, although not known to be prior art with respect to the present invention, is believed to include a support ring which is forced into a groove in the head in a separate operation by a special tool. Again, such a system is less advantageous than the present invention, which does not require any special tools or operations to run and set the hangers in the accompanying head.